Recently, the so-called plasma processing has been actively employed for, e.g., etching and film formation on a semiconductor and a glass substrate. To achieve a desired condition in the plasma processing, it is important to control fundamental quantity (i.e., physical quantity) of plasma itself. For example, efficient plasma processing for purposes such as etching and film formation, will be achieved if a spatial distribution and a time-dependent shift of plasma electron temperature can be measured for plasma, which is generated for executing the plasma processing such as etching and film formation, and plasma electron temperature can be controlled based on the measurement result. In executing the plasma processing, plasma measurement (plasma assessment), i.e., obtainment of the fundamental quantity (i.e., physical quantity) of plasma itself (e.g., plasma electron temperature), is important.
Therefore, a variety of methods of measuring plasma electron temperature, including e.g., the Langmuir probe method and the Thomson scattering method, have been conventionally used. In the Langmuir probe method, however, a metal probe is required to be inserted into plasma. A drawback accordingly is occurred in that metal is polluted by the plasma. Therefore, the Langmuir probe method has not been suitable for plasma measurement in the actual plasma processing (e.g., etching and film formation). On the other hand, a measurement device, having complex structure, is required for the Thomson scattering method. Additionally, the Thomson scattering method is a technically difficult method. Therefore, it has been impossible to make a measurement of the plasma with a simple device configuration at relatively low cost in the plasma processing.
Japan Laid-open Patent Publication No. H10-83893 and Japan Laid-open Patent Publication No. 2006-318800 describe other methods of measuring plasma electron temperature. Japan Laid-open Patent Publication No. H10-83893 describes an example of a method of measuring plasma electron temperature using so-called emission spectroscopy. According to the method of measuring plasma electron temperature described in Japan Laid-open Patent Publication No. H10-83893, measurements are made of light emission of atoms in plasma due to a lower level transition process of the atoms from the first excited state and light emission of atoms in plasma due to other lower level transition process of the atoms from the second excited state. Plasma electron temperature is subsequently computed based on emission intensity ratio between the above the two light emissions. In the method described in Japan Laid-open Patent Publication No. H10-83893, plasma electron temperature is computed based on an assumption that the number of atoms excited at a predetermined energy level in plasma is based on rules of statistical mechanics including the Boltzmann distribution when the plasma is in thermal equilibrium. In the method described in Japan Laid-open Patent Publication No. 2006-318800, on the other hand, laser light, having suitable wavelength enabling to excite atoms of argon gas in a metastable state, is irradiated to plasma of the argon gas which is generated in a chamber for measuring transmittance of the laser light. Density of the atoms in the metastable state is computed based on information of the measured transmittance of the laser light. Moreover, plasma electron temperature is computed in a limited condition, using the computed density of the atoms in the metastable state and the rate equation for determining the metastable state.